1. Field of the Invention
This invention relates to processes for the thermal and hydrolytic stabilization of sulfonated polymers having one or more sulfonic acid groups covalently bonded to one or more carbon atoms in one or more molecules comprising the polymers. Most particularly, this invention relates to the reaction products of the sulfonated polymers with an oxirane to form a hydroxyalkyl ester of the sulfonic acid groups and the conversion, in whole or in part, of the ester to a solid, gel-free ionomer by the reaction of the reaction of the ester with a mono or polyvalent base, or salt of an acid having a lower K.sub.a than the sulfonic acid.
The hydroxyalkyl esters of this invention may be schematically represented as follows: ##STR1## wherein C is one or more carbon atoms in one or more molecules comprising a polymer and is in the polymer backbone-chain of carbon atoms or is in an acyclic, alicyclic, or aromatic group which is pendant to the backbone chain; R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are independently selected from the group consisting of hydrogen, C.sub.1 to C.sub.18 straight and branched chain acyclic, alicyclic, aryl, alkylaryl, arylalkyl groups and functional derivatives thereof.
Products which have a limited concentration of ionic groups covalently bonded to one or more carbon atoms of one or more molecules comprising a polymer, when partially or completely neutralized with mono or polyvalent cations such as sodium, zinc, magnesium, lead, ammonium, and quaternary ammonium compounds are generally referred to as ionomers. The ionic groups can be covalently bonded to carbon atoms comprising the backbone chain of the polymer, which chain of carbon atoms may be interrupted by hetero atoms such as oxygen, nitrogen and sulfur, or the ionic groups can be bonded to acyclic, alicyclic or aromatic groups which are pendant to the backbone chain. Ionomers differ from covalently corss-linked elastomers, such as vulcanized rubber, in that while ionomers can be elastomeric at ordinary temperatures, at elevated temperatures, they can be extruded, compression molded, blow molded, sheeted, vacuum formed and injection molded.
Sulfonated polymers are known to be relatively unstable and are difficult to process in safety or without degradation in conventional plastic or rubber equipment at elevated temperatures. Except for this tendency to degrade it would be advantageous to process the free acid since the weak ionic associations result in a low polymer viscosity permitting easy mixing with neutralizing agents, plasticizers, and other polymers. These systems can then be reacted with the neutralizing agent which is an amine or metallic or quaternary base such as an oxide or hydroxide or salt of a weak acid such as an acetate or stearate to yield the ionomer.
On the other hand, if the sulfonated polymer is first neutralized with a counterion and the ionomeric product isolated, it is found to possess an extraordinarily high viscosity even at elevated temperatures commonly used for processing thermoplastic polymers in conventional equipment and must be mixed with other materials by solution techniques.
A major object of this invention is the preparation of low viscosity, thermally and hydrolytically stable esters of sulfonated polymers which are readily convertible to solid, gel-free ionomers by reaction with a basic substance. A further object of this invention is the preparation of ionomers which comprises reacting a polymer having one or more sulfonic acid groups covalently linked or bonded to one or more carbon atoms in one or more molecules comprising said polymer with an oxirane to form a solid, gel-free hydroxyalkyl ester and reacting said ester with a basic substance selected from the group consisting of mono and polyvalent amines, quaternary ammonium oxides and hydroxides, and mono and polyvalent metallic oxides, hydroxides, alkoxides and salts of acids having a lower K.sub. a than the sulfonic acid. These, and other objects of this invention will be further apparent in the detailed description of the invention with follows below.
2. Prior Art
The reaction of an oxirane (e.g. ethylene oxide) with a sulfonic acid to form a hydroxyethyl ester is old in the art. U.S. Pat. No. 2,208,581 discloses the preparation of surface active agents by the reaction of low molecular weight sulfonic acids with a compound containing an olefin oxide.
In copending application concerning preparation of ionomeric latex epoxides are used to quench the sulfonation reaction and allow the cement to be processable in a unique latex process. However, this copending application fails to teach a method for the isolation of a solid, gel-free, hydrolytically stable polymeric hydroxy alkyl sulfonate having desirable physical and rheological properties, wherein these polymeric sulfonates are readily formable on conventional plastic type processing equipment in a variety of useful articles. Furthermore, it is completely unpredictable that a solid, gel-free, hydrolytically stable polymeric hydroxy alkyl sulfonate is capable of being isolated in a steam stripping operation under acidic conditions and at higher temperatures based upon the published literature on the hydrolytic stability of low molecular weight alkyl sulfonates. No references have been found in the chemical or patent literature for the preparation of hydroxylalkyl esters of polymeric sulfonic acids or for the preparation of ionomers from these esters by reaction with a basic compound or salt of an acid having a lower K.sub.a than the sulfonic acid.